A dual approach to water well drilling

Australia is a dry country wracked with drought, and has very little water. 
This is even more severe in remote areas. 
Unfortunately for mining, many of its operations take place in remote dry areas such as Western Australia's Pilbara, New South Wales' Central West, or South Australia's northern reaches. 
So being able to access water, quicker and more efficiently without the risk of potential contamination during drilling is paramount. 
So Boart Longyear's work in the Pilbara using dual-tube flooded reverse circulation (DTFR) water well drill rigs has drawn attention for their use in well drilling activities. 
Currently Boart Longyear's contractor Water Services Division has multi-year contracts with one of the larger iron ore miners in the Pilbara to carry out water well drilling at its active and future mine sites. 
Typically conventional air drilling or dual air rotary drilling is used to set large diameter water wells, for de-watering, and injection wells. 
While these historically used methods can drill the hole quickly, maintaining circulation and hole integrity can be a serious challenge, especially in weak, fractured, or friable ground conditions. 
Because of this, "as a result of circulation loss or collapsing, [these] boreholes are often completed at shallower depth", Boart Longyear said. 
In order to overcome this issue teams can drill a large diameter hole, which requires either conventional air or dual rotary air that produces a high amount of discharge water. 
"This is an issue because many drill locations can not release water to the environment due to water quality issues, so the water is contained in giant sumps dug parallel to the drill. Often these pits fill quickly during drilling and operations must be stopped for draining," Boart Longyear explained, a process which slows down the process. 
In order to overcome this issue the contract has utilised DTFR rigs.  
According to Boart Longyear this rigs have the ability to accommodate casing diameters of up to 71 centi­­metres, attain deeper drill targets due to a 58 967 kilogram pullback, and are mounted on a high mobile platform. However the aspect that sets them apart is the closed circulation mud system that runs through specially designed mud tanks.  
DTFR distinguishes itself from the other drilling methods by the way the drill cuttings are returned to the surface through the drill string, as opposed to the annulus. 
"By installing de-watering wells via DTFR, it is possible to control high producing aquifers during drilling that are problematic for conventional mud or conventional air drilling," the contractor said. 
As the cuttings are returned via the string and don't mix with the annulus there is reduced chance of contamination, annulus washouts, and hole enlargement. 
Because the samples are returned via the string mud loss will not stop drilling operations as long as the bit remains submerged. This means circulation is almost never lost and drilling mud can be replaced during operation without the need to stop. 
By utilising the proprietary closed loop mud system the system's footprint is greatly reduced, and formation water is not an issue during drilling as there is almost zero discharge during operations, which makes water management a non-issue. 
Over the space of a year Boart Longyear used this method to drill 2300 metres for the Pilbara miner, with hole diameters of 44.45 centimetres to 52 centimetres, with completed wells reaching depths of 250 metres and were cased with 25 to 31 centimetres production strings. 
On one well it achieved a depth of 144 metres with less than one per cent of deviation during drilling, this was despite encountering geology such as banded iron, thick sequences of friable mineralised ore, and shale inter-bedded with hard chert bands. 
Prior to using DTFR there was speculation that a tri-cone drill bit, which is used in mud drilling, wouldn't able to handle the hard rock typically associated with iron ore deposits, however it was capable of drilling in these conditions.

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